The waste product of papermills, commonly called sludge, comprises approximately 98% moisture. A belt press may be used to press some of the moisture out of the sludge before disposal but the waste material taken to the landfill still often contains as much as 70% moisture. Since landfills generally charge by the pound, it is consequently very expensive to dispose of the sludge.
The prototype gravity fed dryer shown in U.S. Pat. No. 4,888,885 (incorporated herein by this reference) was designed to dry this type of high water content material but, for a number of reasons, was found to be unsuitable for inclusion into a commercial paper manufacturing facility.
First, a typical paper plant facility often operates and therefore generates sludge 24 hours a day. The gravity fed system shown in the '885 patent requires manpower to feed the dryer and discharge the dried material. Second, the quantity of sludge produced by a commercial paper mill varies over time. Thus, without some means of assuring a constant feed to the dryer shown in the '885 patent, there will be times that the dryer is not working efficiently or not drying material at all requiring inefficient shut down/start-up cycles. Worse, there may be times when the dryer cannot handle the large amount of material fed to it.
Third, the quality of sludge (fiber, filler, and moisture content) produced by the mill varies over time. The paper mill may be set up to produce a high quality bond paper one day, cigarette paper the next day, and a cardboard type paper at other times. The sludge from different types of paper has different qualities. In the '885 patent disclosure, there is no way to control the operating parameters of the dryer (e.g., processing time, air temperature, pressure heads), and consequently it was found unsuitable for use in actual commercial paper mills. Use of the dryer shown in the '885 patent resulted in an end product which may not be dry enough, or too dry resulting in a waste of energy.